1. Field of the Invention
The present invention relates generally to a process for the removal of mercaptans and/or hydrogen sulfide (H2S) from petroleum distillate streams. More particularly the invention relates to a process wherein the petroleum distillate contains diolefins which are selectively reacted with the mercaptans and/or hydrogen sulfide (H2S) to form sulfides. Most particularly the invention relates to a process wherein the reaction of the mercaptans and/or hydrogen sulfide (H2S) with the diolefins is carried out simultaneously with a fractional distillation to remove the sulfides, and thus the sulfur, from the distillate.
2. Related Information
Petroleum distillate streams contain a variety of organic chemical components. Generally the streams are defined by their boiling ranges which determine the compositions. The processing of the streams also affects the composition. For instance, products from either catalytic cracking or thermal cracking processes contain high concentrations of olefinic materials as well as saturated (alkanes) materials and polyunsaturated materials (diolefins). Additionally, these components may be any of the various isomers of the compounds.
The petroleum distillates often contain unwanted contaminants such as sulfur and nitrogen compounds. These contaminants often are catalyst poisons or produce undesirable products upon further processing. In particular the sulfur compounds can be troublesome. The sulfur compounds are known catalyst inhibitors for naphtha reforming catalysts and hydrogenation catalysts. The sulfur compounds present in a stream are dependent upon the boiling range of the distillate. Light naphtha (110-250xc2x0 F. boiling range) may contain mercaptans as the predominant sulfur compounds. The most common method for removal of the H2S and mercaptans is caustic washing of the organic streams.
Another method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally copious quantities of hydrogen are included in the feed. The following equations illustrate the reactions in a typical HDS unit: 
Typical operating conditions for the HDS reactions are:
As may be seen the emphasis has been upon hydrogenating the sulfur and other contaminating compounds. The sulfur is then removed in the form of gaseous H2S, which in itself is a pollutant and requires further treatment.
In the production of tertiary amyl methyl ether (TAME) for use as a gasoline additive generally a light cracked naphtha (LCN) is used as the source of the olefins for the etherification reaction. This LCN may contain sulfur as a contaminant in the form of mercaptans in concentrations of up to hundreds wppm. These mercaptans are inhibitors for the hydrogenation catalyst used to hydrogenate dienes in the feed to an etherification or to an alkylation unit. As noted above, one common method has been caustic washing.
The present invention presents a new process for the removal of mercaptans and/or hydrogen sulfide (H2S) from aliphatic hydrocarbon streams, containing 4 to 12 carbon atoms. Light cracked stream which is used as a feed to an etherification or alkylation unit is a preferred feed for this process. The light cracked naphtha contains C4""s to C8""s components which may be saturated (alkanes), unsaturated (olefins) and poly-unsaturated (diolefins) along with minor amounts of the mercaptans. The light naphtha is generally depentanized in a fractional distillation column to remove that portion containing the C6 and higher boiling materials (C6+) as bottoms and the C5 and lower boiling materials (C5xe2x88x92) as overheads. One embodiment of the present invention utilizes the upper portion of the depentanizer to react substantially all of the mercaptans and/or hydrogen sulfide (H2S) contained in the light cracked naphtha with a portion of the diolefins to form sulfides which are higher boiling than the C5 fraction containing the amylenes which are fed to the etherification and/or alkylation unit. The sulfides are removed as bottoms from the depentanizer column along with the C6+ fraction and can be simply remixed into the final gasoline fraction.
The catalyst used for the reaction is a reduced nickel, preferably 5 to 70 wt % nickel, such as nickel sulfide on an alumina base which has been configured as a catalytic distillation structure.
Hydrogen is provided as necessary to support the reaction. The distillation column reactor is operated at a pressure such that the reaction mixture is boiling in the bed of catalyst. A xe2x80x9cfroth levelxe2x80x9d may be maintained throughout the catalyst bed by control of the bottoms and/or overheads withdrawal rate which may improve the effectiveness of the catalyst thereby decreasing the height of catalyst needed. As may be appreciated the liquid is boiling and the physical state is actually a froth having a higher density than would be normal in a packed distillation column but less than the liquid without the boiling vapors.
The present process preferably operates at overhead pressure of said distillation column reactor in the range between 0 and 250 psig and temperatures within said distillation reaction zone in the range of 100 to 300xc2x0 F., preferably 130 to 270xc2x0 F.
The feed and the hydrogen are preferably fed to the distillation column reactor separately or they may be mixed prior to feeding. A mixed feed is fed below the catalyst bed or at the lower end of the bed. Hydrogen alone is fed below the catalyst bed and the hydrocarbon stream is fed below the bed to about the mid one-third of the bed. The pressure selected is that which maintains catalyst bed temperature between 100xc2x0 F. ad 300xc2x0 F.